1. Field of the Invention
The present invention relates to a vacuum pump for evacuating a vacuum chamber, and more particularly to an improvement of the vacuum pump in which a rotor arranged inside has a threaded portion on the surface thereof.
2. Description of the Related Art
Vacuum pumps are used, for example, as vacuum producing apparatus for exhausting gas within a chamber of semiconductor manufacturing equipment, so at to discharge a process gas supplied to the chamber for processing the semiconductor devices therein.
FIG. 5 shows the overall structure of the vacuum pump. In FIG. 5, reference numeral 101 denotes a casing in which a gas inlet port 102 and a gas outlet port 103 are formed. A rotor 104 is housed in the casing 101. Formed on this rotor 104 are a plurality of rotor blades 105 projecting outwardly in a radial direction toward the inner circumferential wall of the casing 101, and a threaded portion 108 disposed below the rotor blades 105 and having spiral thread grooves formed therein.
A plurality of stator blades 106 and a stator 109 are attached to the inner circumferential wall of the casing 101 facing the rotor blades 105 and the threaded portion 108, respectively. The rotor 104 is rotated by a motor 107 housed in the casing 101, which causes the rotor blades 105 and the threaded portion 108 to rotate at a high-speed relative to the stator blades 106 and the stator 109, respectively.
A stator blade 106 and a stator 109 are attached onto the inner circumferential wall of the casing 101 while facing with the rotor blade 105 and the threaded portion 108, respectively. The rotor 104 is rotated by a motor 107 housed in the casing 101, which causes the rotor blade 105 and the threaded portion 108 to rotate at a high-speed relative to the stator blade 106 and the stator 109, respectively.
The rotor 104 is fixedly provided with a rotor shaft 112 and is rotatably floated by magnetic force produced by an axial electromagnet 113 and a radial electromagnet 114. Further, touch down bearings 115 and 116 are provided in an outer member of the rotor shaft 112 so as to come in contact with the rotor shaft 112 and to rotatably support the same in the case where the rotor shaft 112 is floated, but is not supported through magnetic force by the electromagnets 113 and 114.
However, a conventional vacuum pump constructed as described above has structural defects. As shown in FIG. 6, a terminal end face, which is located on the downstream gas suction side (lower end in the drawing), of a thread 108a in the threaded portion 108 is formed so as to be identical with the end face of the rotor 104 on the downstream gas suction side (lower end in the drawing). A thread groove 108b is formed axially between two adjacent lines of thread 108a, which is formed by machining with an edge tool to have a sharply gouged bottom corner. Such structure causes the centrifugal force upon rotation of the rotor 104 to tend to concentrate stress on the bottom corner of the thread groove 108b.
In particular, a bottom corner C (see FIG. 7) of the thread groove 108b, located at a terminal B of the thread 108a in FIG. 6, is at a location at which the edge tool is pulled out upon completing the machining. Accordingly, a notch is liable to be produced due to imbalance in machining resistance. For this reason, the bottom corner C is liable to start a crack to eventually damage the rotor 104 with the centrifugal force upon rotation.
To solve the above-mentioned problem, there is known a vacuum pump having a rotor in which the spiral thread is provided on the surface of the rotor so as to project with a thread groove that is axially formed between two adjacent lines of the thread. In this vacuum pump, the position of the terminal end face of the thread on the downstream gas suction side is shifted so that it becomes shorter than the end face of the rotor on the downstream gas suction side, and a recessed R portion is formed at the root of the terminal end face of the thread on the downstream gas suction side.
FIG. 8 is a side view of the end portion of the rotor. In FIG. 8, reference numeral 4 denotes the rotor of the vacuum pump. A spiral thread 4a is formed projectingly on the surface of this rotor 4 so that a thread groove 4b is formed axially between two adjacent lines of the thread 4a. At the lower end portion of the rotor 4 in the drawing (the end portion on the downstream gas suction side), the position of a terminal face 40 of the thread 4a on the downstream gas suction side is shifted so as to be shorter by a length H than the position of an end face 41 of the rotor 4 on the downstream gas suction side.
For this reason, at a bottom corner of the thread groove 4b at the end portion of the downstream gas suction side of the thread 4a, even if a notch is produced when an edge tool is pulled out upon completing the machining, due to imbalance in machining resistance, if, thereafter, the end portion of the thread 4a on the downstream gas suction side is scraped by H so that the downstream gas suction side terminal face 40 of the thread 4a is shifted to position to reach short of the downstream gas suction side end face 41 of the rotor 4, it is capable of scraping off the notch, too, caused by the imbalance of machining resistance, thereby being capable of, unlike conventional pumps, preventing formation of a crack developed from the notch, which may cause damage to the rotor with the centrifugal force upon rotation.
In the vacuum pump with such a structure, the downstream gas suction side terminal face of the thread is shifted so as to reach short of the end face of the rotor on the downstream gas suction side. Therefore, even if a notch is produced at the bottom corner of the thread groove at the downstream gas suction side terminal face of the thread by an edge tool pulled out upon completing the machining, due to imbalance in machining resistance, if, thereafter, the end portion of the thread on the downstream gas suction side is scraped a little so that the downstream gas suction side terminal face of the thread is shifted to reach short of the downstream gas suction side end face of the rotor, it is capable of scraping off the notch, too, caused by the imbalance of machining resistance, and further, by finishing the root of the downstream gas suction side terminal of the thread into a shape of a recess R, it is capable of preventing the concentration of stress on the root, thereby being capable of preventing damage to the rotor due to a crack developed from the notch by the centrifugal force upon rotation.
However, even this device, having the thread near the rotor end, is not free from a problem of concentration of bending stress at the thread terminal, which is stress concentration on the thread root caused by a difference in thickness between the portions with thread and without thread.